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Abstract
Quantum bits (qubits) are the basic building blocks of any quantum computer. Superconducting qubits have been created with a top-down approach that integrates superconducting devices into macroscopic electrical circuits, and electron-spin qubits have been demonstrated in quantum dots. The phase coherence time (τ2) and the single qubit figure of merit (QM) of superconducting and electron-spin qubits are similar -- at τ2 ∼ µs and QM ∼ 10-1,000 below 100 mK -- and it should be possible to scale up these systems, which is essential for the development of any useful quantum computer. Bottom-up approaches based on dilute ensembles of spins have achieved much larger values of τ2 (up to tens of milliseconds; refs 7,8), but these systems cannot be scaled up, although some proposals for qubits based on two-dimensional nanostructures should be scalable. Here we report that a new family of spin qubits based on rare-earth ions demonstrates values of τ2 (∼50 µs) and QM (∼1,400) at 2.5 K, which suggests that rare-earth qubits may, in principle, be suitable for scalable quantum information processing at 4 He temperatures.